Zinc electroplating process and acidic zinc fluoborate electrolyte therefor

ABSTRACT

An acidic zinc electroplating bath comprising zinc fluoborate dissolved in an aqueous liquid containing as an iron- and metal oxide-deposition inhibitor a small effective amount of a mixture of a 4-hydroxy-, 4-alkyl- or aryl-, 5-acyl- or aroyl-piperidine and thiourea or N-substituted derivative thereof. The invention also includes an improved process for zinc-electroplating ferrous metals from such bath.

United States Patent Page et al.

[151 3,655,533 [451 Apr. 11, 1972 [54] ZINC ELECTROPLATING PROCESS AND ACIDIC ZINC FLUOBORATE ELECTROLYTE THEREFOR [72] lnventors: Walter Page, Convent, N.J.; William R. Schevey, l-lawley, Pa; John E. Vander Mey, Stirling, NJ.

[73] Assignee: Allied Chemical Corporation, New York,

[22] Filed: Nov. 12, 1970 [21] Appl. No.: 89,068

[52] U.S. Cl. ..204/55 R [51] Int. Cl. ...C23b 5/12 [58] Field of Search ..204/55 R [56] References Cited UNITED STATES PATENTS 2,523,160 9/1950 Struyketal ..204/45 2,537,032 l/ 1951 Chester et al. ..204/55 R 2,807,585 9/1957 Gardner et al ..252/ 148 FOREIGN PATENTS OR APPLICATION S 1,109,479 6/1961 Germany ..204/55 R 254,292 3/1970 U.S.S.R ..20/55 R Primary Examiner-F. C. Edmundson Attorney-Jay P. Friedenson [57] ABSTRACT 21 Claims, No Drawings BACKGROUND OF THE INVENTION This invention relates to a process for electrodepositing zinc from acidic zinc fluoborate plating baths and, morepa'rticulady, to a procedure for plating zinc on ferrousobjects which inhibits co-plating of iron and deposition of iron and zinc oxides. The invention also includes a novel acidic zinc fluoborate bath'for use in this process.

it is known to plate zinc onto ferrous metal objects, such as steel or iron rods, wires, strips and the like, from acid zinc fluoborate plating baths by directing an electric current "through such electrolyte bathsusing zinc metal as the anode and the steel object to be plated as the cathode. One typical acidic zinc fluoborate plating electrolyte has been comprised of zincfluoborate in aqueous solution, usually also containing coarse-grained in texture, rather than smooth and fine 'grained as is desirable: Furthermore, the strongly corrosive fluoboric acid of the bath dissolves the ferrous metal cathode resulting in build-up of cationic iron in the electrolyte. The dissolved ironthen co-plates with the zinc on the cathode resulting in an undesirable mixed plating of zinc and iron. Zinc plated ferrous metal articles, for example, steel strip, are generally fabricated into useful articles by cutting, stamping, drawing and forming operations. In order to withstand such operations without 7 damage to the adhesion of the coating to the steel surface, it is essential that the electroplated zinc coating be ductile, tightly adhering and that it have a relatively low content of impurities, particularly iron co-deposited with zinc.

An additional disadvantage of conventional acidic zinc fluoborate electroplating baths and processes also arises from aforementioned build-up of cationic'iron in the electroplating electrolyte. It is found that conventional acidic zinc fluoborate electroplatingbaths have a low tolerance for dissolved cationic iron. At electrolyte concentrations of cationic iron exceeding a certain low limit, say about 4 g. iron per liter of bath,

the dissolved iron interferes with the normal plating of zinc resulting in a black spongy deposit of zinc andiinc oxide containing a small amount of iron oxides on the surface of the cath'ode, instead of the attractive white zinc coating normally obtained. As will be apparent to those skilled in the art, addi- *tion tothe plating bath of a conventional iron anticorrosion 1 agent; that is,- an agent which inhibits corrosion of iron or steel by acids, is not an adequate remedy to the foregoing problems associated with dissolved cationic iron build-up, since-such agents merely retard, rather than prevent, dissolution of iron by acidic substances.

It is,"therefore, an object of the present invention to provide a process for electroplating zinc on ferrous rnetalsfrom an acidic zinc'fluoborate electrolyte which minimizes the co- "deposition of iron with the zinc on the ferrous metal cathode.

"It" is a further object of the invention to provide an acidic 'zinc'fluoborateelectroplating bath which tolerates larger than normal concentrations of cationic iron in the bath before unacceptable deposition of metal oxides occurs.

It is a still further object of the invention to provide an im- "proved process and electrolyte for electroplating zinc on ferrous objects in a desirable fine grain of attractive appearance.

These and otherobjects and advantages will be apparent "from the following description of our invention.

SUMMARY'OF THE INVENTION The aforementioned disadvantages of prior an acidic zinc fluoborate electroplating baths and processes are overcome and the above objects are'attained according to our invention wherein an acid zinc fluoborate electroplating bath is provided'comprising zinc'fluoborate dissolved in an acidic aqueous liquid containing'a mixture of a 4-hydroxy-, 4-alkyl-' or 'aryI-, 5-acyl or aroyl-piperidine and thiourea or N-alkyl-,

arylor aralkylsubstituted derivatives thereof in a small amount capable'of promoting formation of an attractive finegrained zinc coating, of inhibiting iron plating and of raising the tolerance of the bath toward dissolved iron so as to forestall cathode deposition of zinc and iron oxides.

Itwas highly surprising to discover, according to the invention,'that the presence of mixtures of the aforementioned substituted piperidines and thioureas effectively inhibited the co plating of iron with zinc on electroplating ferrous metals from acidic aqueous zinc fluoborate baths, as is illustrated below in Example 4.-Such piperidine-thiourea mixtures have been disclosed by U.S. Pat. No. 2,807,585 as anti-corrosion agents for ferrous metals, that is, as inhibitors to retard dissolution of such metals by aqueous acids, which dissolution is an oxidation reation. It was, therefore, surprising to discover that such mixtures in an electroplating environment inhibited the reverse reaction, namely, the electrochemical reduction of cationic iron to the free ferrous metal.

It was also surprising to discover, in accordance with the invention, that the present additive increases the tolerance of acidic aqueous zinc fluoborate electroplating baths toward dissolved cationic iron by as much as about 300 percent, as is illustrated-below in Example 3. Unexpectedly also, the zinc electroplatings on iron or steel obtained from an aqueous acidic zinc fluoborate electrolyte containing the present additive are of substantially finer grain and more attractive appearance than the corresponding zinc platings obtained from conventional aqueous acidic zinc fluoborate electrolytes devoid of this additive.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS THEREOF The substituted piperidine' additives of the invention have the structural formula o l c and R is lower alkyl, aryl of the benzene or naphthalene series or aralkyl in which the alkylene moiety is methylene or vinylene and in which the aryl moiety is of the benzene or naphthalene series.

The designations lower alkyl and lower cycloalkyl, employed in the above definition and hereinafter, refer to saturated radicals containing up to eight carbon atoms. The designation of aryl radicals or aryl moieties as being of the benzene or naphthalene series, as employed in the above definition and hereinafter, refers to phenyl or naphthyl radicals which may be unsubstituted or which contain, in addition to hydrogen, other conventional inert substituents such as lower alkyl, halogen, such as fluorine, chlorine and bromine, and lower alkoxy. By inert, we mean substituents which are inert to the acidic plating bath under electroplating conditions.

Typically, R of the above general structural formula (I) can be methyl, n-propyl, isopropyl, n-butyl, t-butyl, n-hexyl, n-octyl, benzyl, p-chlorobenzyl, l-naphthylmethyl, cyclohexyl or cyclopentyl. Typically, R can be methyl, ethyl, n-propyl, isopropyl, t-butyl, n-hexyl, n-octyl, phenyl, lor Z-naphthyl, 5 benzyl, lor Z-naphthylmethyl, 0-, mor p-anisyl, 0-, mor pchlorophenyl, o-, mor p-chlorobenzyl 0r o-,m-or p-tolyl.

The substituted piperidines charged in accordance with the invention are known compounds disclosed in U.S. Pat. No. 2,807,585 and .l. T. Pati et al., J. Org. Chem. 14 873(1949), the pertinent subject matter of which patent and article are incorporated herein by reference.

Preferably, we employ a substituted piperidine of the above general formula in which R is a The N,N-substituted thiourea charged in accordance with the invention has the following structural formula wherein R and R are individually selected from lower alkyl radicals, lower cycloalkyl radicals, aryl radicals of the benzene or naphthalene series and aralkyl radicals wherein the alkylene moiety is methylene and the aryl moiety is of the benzene or naphthalene series.

Typical suitable thiourea additives include the following representative examples:

thiourea N-methyl thiourea N-phenyl thiourea N-p-tolyl thiourea N-benzyl thiourea N-( l-naphthyl)thiourea N-( Z-naphthyl )thiourea N-(2-naphthylmethyl)thiourea N-(n-octyl) thiourea N-(n-isopropyl)thiourea N-cyclohexylthiourea N,N'-dicyclohexylthiourea N,N'-dicyclooctylthiourea N-cyclopentylthiourea N,N-dicyclopentylthiourea N,N'-(p-anisyl)thiourea N-(phenyl)N'-(p-tolyl)thiourea N-(methyl)N'-(benzyl)thiourea N,N-di(p-chlorobenzyl)thiourea N,N-di(p-isopropylphenyl)thiourea N,N-di)p-t-butylphenyl)thiourea N,N-di( l-naphthyl)thiourea N-phenyl-N'( l-naphthyl)thiourea N-methyl-N-ethyl thiourea N,N-di( l-methylnaphthyl)thiourea N,N-di( l-naphthylmethyl) thiourea N,N-di(p-fluorophenyl)thiourea N,N-di(m-tolyl)thiourea N,N-di( l-naphthyl)thiourea N,N'-di(2,4-dichlorobenzyl)thiourea N,N-di(t-butyl)thiourea N,N-di(p-bromobenzyl)thiourea N,N-di(o-chlorophenyl)thiourea N,N-di(p-fluorophenyl)thiourea Mixtures of these and equivalent thiourea compounds corresponding to the above general structural formula (III) may also be employed in the additive formulation of the invention.

Preferably, we employ a thiourea wherein R and R are both aryl radicals of the benzene series. Use of N,N'-di)ptolyl)thiourea as the thiourea additive provides an especially good result.

The concentration of the mixture of substituted piperidine and thiourea compound which is required in the zinc fluoborate plating bath of the invention to inhibit iron plating, to produce a fine-grain zinc coating and to increase the tolerance of the electrolyte toward dissolved iron can be as small as about 0.004 g. per liter of the bath or less and preferably is about 0.03 to 0.2 g. per liter of bath. A concentration of 10 g. per liter or more of the substituted piperidinethiourea mixture, however, can be employed in accordance with the invention.

The proportion of the substituted piperidine compound and thiourea compound in the mixture charged to acidic zinc fluoborate electroplating baths, in accordance with the invention, can vary over a wide range. Thus, there can be employed as additive, mixtures containing from about 20 weight percent to about weight percent of the substituted piperidine, the balance of the mixture being the thiourea compound. Preferably, the additive contains about 50 to 75 weight percent of the substituted piperidine compound.

The aqueous zinc fluoborate electrolyte to which the inhibitor mixture is charged, in accordance with the invention, is conventional in the electroplating art. A preferred typical aqueous acid zinc fluoborate electrolyte to which the inhibitor mixture is charged in accordance with the invention has the following composition:

Ingredient Dissolved in the Bath zinc fluoborate ammonium chloride ammonium fluoborate ammonium hydroxide Other suitable conventional aqueous acidic zinc fluoborate electrolyte formulations for electroplating zinc on ferrous metals in the presence of the above-described inhibitor mixture will be apparent to those skilled in the art.

The electrogalvanizing process of the invention can be carried out employing a zinc fluoborate bath of the abovedescribed type wherein the inhibitor mixture is dispersed or advantageously dissolved. A zinc anode is inserted in the bath and the ferrous metal object to be plated is made the cathode. In accordance with conventional electroplating procedures, the temperature of the bath during plating can conveniently range from about ambient temperature to about 200 F but advantageously is about to 190 F., especially about 180 F. Desirably also, the bath is well-agitated during the electroplating operation. Zinc electroplating, in accordance with the invention, can be carried out batchwise or in a continuous fashion.

aforementioned blackening of the anode and does not completely eliminate, precipitation delivered tothe zinc anode,

minates the plating operation. As is apparent from the results of Example l(b) below, plating from zinc fiuoborate baths at anode current densities above about 700 asf eliminates the minimizes, but of zinc hydroxyfluoborate, To eliminate the, latter undesirable effect, in accordance with a particularly preferred mode of operation, about 1 to 5 percent, advantageously about 2 to 5 percent, of the aforementioned high density current, which is normally is shunted, variably if desired,

through an inert carbon anode inserted in the bath while maintaining the overall bath efficiency during the plating operation 3 at about 100 percent, that is at 100 1- 3 percent, advantageously at 100;- 1 percent. Current shunting, which can current densities, say about 5 to 7 00 asf, causes rapid corrosion of the zinc anode which becomes respectively, in the zinc electroplating of steel from a conventional aqueous acidic zinc fluoborate bath.

EXAMPLE 1 Two zinc-plating baths, (a) and (b), are prepared having the following composition:

Ingredient Grams/Liter of Aqueous Solution Equivalent to 8| grams ofzinc pci' liter.

The pH of each bath is adjusted to 3 to 3.5 by addition of ammonium hydroxide. An anode of zinc metal and a cathode of strip steel are inserted into each bath. In order to determine the efficiencies of the anode and cathode in the above baths when operating at various current densities, a current of 2,800 coulombs is passed through each bath, the current through bath (a) having a density of 250 amperes per square foot of anode, the current through bath (b) having a density of 750 amperes per square foot of anode. Amounts of zinc released from the anodes and electrodeposited on the cathodes are measured and compared with the theoretical values calculated for these conditions. The temperature of the plating baths during the electroplating operation is 180.

Results, which represent an average of three replications at each current densit are shown in Table I below:

TABLE I.--Efiieiencies of zinc fiuoborate plating baths at 250 asf and 750 asr current dcnsity- 1 The zine anode rapidly corroded and developed a black zinc oxide coating as the bath H e to 4. On continued operation at these conditions, a heavy sludge of zinc hydroxyfiuoborate precipitates as the path pH rises to above 4. This sludge migrates to the cathode forming aloosel 2 The zinc an diminished de y adherent layer thereon. terminating the plating process.

ode remains clear of oxide deposits as the bath pH rises to 4, indicating at area of anode corrosion. On continued operation pH 4, some zine hydroxyiluoborate precipitates.

be carried out in a continuous or semi-continuous manner during plating, maintains the concentration of zinc ion dissolved in the bath within about 10 weight percent of the initial zincconcentration of the electrolyte, thereby preventing any substantial accumulation of zinc ion in the bath and resultant undesirable precipitation of zinc hydroxy-fluoborate. Since only about 9599 percent of the current normally charged to the zinc anode passes through the zinc anode during current shunting, the corrosion of the latter anode is minimized also.

The present electrogalvanizing process employing the aforementioned preferred process modifications of high density current and current shunting is especially suitable for highspeed continuous zinc plating of steel strip, sheet, wire and the like to obtain platings of improved texture, appearance and diminished co-plated iron content.

in the following examples, which serve to illustrate our invention, temperatures are in degrees Fahrenheit.

The following Examples 1 and 2 illustrate the effects of low and highcurrent density and the effect of current shunting,

EXAMPLE 2 In two experiments, (a) and (b), the procedure of Example 1 is repeated substantially as described, employing a current having a density of 750 asf, except that in each experiment different portions of the anode current are shunted through an inert carbon anode inserted in the bath. In the first experiment (a), 2 percent of the anode current is shunted through the carbon anode; in the second experiment (b), 5 percent of the anode current is shunted through the inert carbon anode.

Resulting efficiencies of these experiments are tabulated in Table II below:

TABLE II.-EFFICIENCIES OF ZINC ANODE, CATHODE AND BATH WHEN SHUNTING 2% AND OF CURRENT THROUGH CARBON ANODES WHEN OPERATING AT 750 A815.

Percent Thcoret- Zine Ellieurrent icalwt. of anode, Efli- Cathode, eieney Bath shunted to zinc to be actual cieney actual eIIiearbon transwt. loss, of anode, wt.gain, cathode, cieney, Experiment anode [erred grams percent grams percent percent I No zinc hydroxytluoborate precipitates.

it is apparent from Table II that the bath efficiencies are TABLE Ill substantially increased by shunting up to 5 percent of the cur- I I I rent normally passed through the zinc anode through an inert Zmc Electroplatmgs from Acldlc Aqueous carbon anode inserted In the bath. As ls further ind cated by Zinc Fluoborate Baths ofGwup X Containing Table ll, the anode and cathode efficiencies of experiment (a) are substantially balanced when about 2 percent of the current 0,075 Grams per Liter of Substituted at 750 asf is shunted through the inert carbon anode with I I I about 98 percent passing through the zinc anode, the overall P ip' n Addmve bath efficiency being 101.0 percent, that is, about 100 percent. In experiment (b) wherein 5 percent of the current is f I shunted, the bath efficiency is considerably above 100 pergrams per Character? Ham's Bath liter of Bath on Steel Cathode cent, that 15, substantially more Zll'lC 18 being plated out on the cathode than is being released into the bath from the zinc I f anode X, l whiternfetqlhc finis IIndIcatIve o l i S3 is ac ory Zn: [3 a [fig It IS apparent from the foregomg experiments that by mx, 2 as in x, above creasing or decreasing the proportion of current shunted 3 Xflbm m hth n b d m t d' "h 5 roug elne car on ano es, eopera or can imlms or X? 8 xuabm increase the zinc ion concentration in the bath and thereby x, to as in x, above maintain said concentration within about 10 percent of the 12 asin X11 4 I I I concentration of zinc ion initially charged. s: 32 i:z g gfgfi'firzzg a l 0' The following Examples 3-20 illustrate the novel acidic m;c,he,wnhzim aqueous zinc fluoborate bath of the invention containing a mixture of a substituted piperidine and thiourea compound described above and the use of this novel bath in electroplat- TABLE Iv ing zinc on ferrous metals.

EXAMPLE 3 Zinc Electroplatings from Acidic Aqueous A series of eight baths as described in example 1 are Zinc Fluoborate Baths of GroupYDevoid prepared, and to each is added 0.25 gram per liter of a com- I I I I I I mercial mixture containing about 70 weight percent of inert ofsubstlmted plpendmeNtN inorganic salt filler and about 30 weight percent of a mixture Thiourea Additive of about 66.7 weight percent of a substltuted piperidme corresponding to the structural formula (I) above and about 33.3 weight percent of N,N-di(p-tolyl)thiourea (Rhodine 115, oramspefiper characmofplaflng American Chemical Paint Co.). This group of baths 18 Bath liter of Bath on Steel Cathode designated Group X. Another group of five baths, designated Grou Y, identical to those of Grou X exce t that no Y,, 1 white metallic plating indicative of P. P 0 plperidine-thiourea mixture is added, iS prepared. Cationic Y 2 fatYisfacbtovry ZlnC plailng iron in the form of ferrous fluoborate 15 charged to each of the 3 Y, g z baths in Group X in varying amounts from 1 to 15 grams of Y, 4 as in Y, above Fe per liter, as indicated in Table III below. In a similar 5 black qes y fi jf 92" manner, ferrous fluoborate is charged to each of the baths in 3:322:35, as

Group Y to provide bath concentrations of Fe varying from 1 to 5 grams per liter, as indicated in Table IV below. After insertion of zinc anodes and steel cathode into each bath, both anodes and cathodes being of the same surface area, currents are passed through each bath at a current density of 1,000 amperes per square foot of anode, at a temperature of 180, and at an initial bath pH OF 3.5. The results of plating from Group X baths and from Group Y baths are set forth in Table Ill and Table IV, respectively.

troplating from such baths in the absence of the additive, at a bath at about 180 at an anode current density of 1,000 asf concentration of more than about 4 grams per liter Fe in the while about 2 percent of the current is continuously shunted bath results in a black deposit of metal oxides at the cathode, through the carbon anode, in accordance with the technique :that is,-prevents obtainment of a satisfactory zinc plating. of Example 2, to maintain the zinc ion concentration in the bath at about 81 i 5 grams per liter. Passage of the current is EX M continued for 10.5 seconds at a cathode current density of .meacidic;aqueous zinc fluoborate. plating baths (a), (b) 7 1,000 asf to obtain a plating of Bi; ounce of zinc per square foot and (c) of thecomposition described in Example '1 are on a h h t h zmc Platmg Smoolh, prepared. Tobaths (a) and (b) isadded 0.25 gram per liter gramed and attractive m appearance (corresponding to 0.075 gram per liter of bath, of active inof the substituted piperidine--N,N-di(p- 1tolyl)theoureaamixtureof Example 3.Ferrous fluoborate is achargedto each .bath to provide the concentrations of Fe indicated in Table V below. Zinc anodes and nickel cathode are inserted in each bath, nickel cathodes being employed in place -.offerrous metal cathodes 'in order to facilitate removal of iresultantzincplatings for analysis'Cu'rrents of 1,000 amperes per square foot of anode are passed through each bath for 30 minutes. The resultant platings are thenstripped from each cathode and analy zed'for total iron content. The results of theseexperiments are presented in Table V below.

EXAMPLE 6 The procedure of Example 5 is repeated substantially as described, omitting the addition of the substituted-piperidinesubstituted thiourea mixture to the'bath. The resultant zinc plating is rough, coarse-grained and unattractive in appearance.

EXAMPLE 7 The procedurecf Example 5is repeated substantially as described, except that'the electroplating bath contains 0.1 gram per liter of l-acetylethyl-4-hydroxy 4-methyl-5-acetyl piperidine having the structuralformula (ll) above and 0.1 gram per liter of N,N'-di(p-tolyl)thiourea in place of the inhibitor mixture charged in Example 5. An excellent plating of TABLE V Concentration orsub. niti c zinc on steel substantially similar to the product of Example 5 "stitutedPiperidinecentration of Tom in is obtained. glfl duptgliy llf Bath 'f In the following Examples 8-20, which are summarized in """T .F (W5 Table VI below, excellent electroplatings of zinc on ferrous Bath Bath (grams per liter) liter) percent) metals are-obtained utilizing the plating technique of Example 7 with various other substituted piperidine and thiourea addi- (a) 0.075 5.9 0.120 0075 no 0178 tives, the structures of which are listed in each tabulated ex- 0 5.9 0.202 ample.

TABLE VI Substituted giperldine of general Thiourea of general structural "structural ormula (1) above Formula (III) above R= R1= Rz= R3= Example:

8; 0Hz0H-=h3-Ri C2H5 Benzyl Benzyl 9 Cyelohexyl --CH5 CH3 c2115 10... CHa Phenyl n-Oetyl H 11... CeH4.CH=CH Cyclohexyl Cyclohexyl 12. n-Butyl Phenyl H Isopropyl Benzyl Phenyl Isopropyl Isopropyl '-CH2OH2OR1 Isopropyl Phenyl Phenyl 15 CHzCH2C-Ri n-Oetyl l-naphthylmethyl l-naphthylmethyl l-naphthyl ,p-Tolyl H 2-naphthylmethyl p-Chlorobenzyl p-Chlorobenzyl Benzyl p-Xylyl p-Xylyl p-Tolyl EAnisyl gl-Anisyl p-Chlorophenyl I We claim:

1. A zinc electroplating bath comprising zinc fluoborate dissolved in an aqueous acidic liquid containing as an ironand metal-oxide deposition inhibitor a small effective amount of a mixture of a. a piperidine of the formula:

It is. apparent from Table V that the presence of the substituted.piperidine-substituted thiourea additive reduces the amount of. iron co-plated with zinc by more than about 40 weightpei'cent when the zinc fluoborate plating bath contains 5.9'gramsper liter dissolved cationic iron.

EXAMPLE 5 30 liter zincfluoborate plating bath of the composition desc-rib'edin Example, 1 is prepared. To the bath is charged the 0 C substituted piperidineN,N '-di(p-tolyl)thiourea mixture of Example3 in an amount equivalent to 0.25 gram per liter of bath. (corresponding to 0.075 gram per liter active ingledients). A zincanode; an inert carbon anode and a steel f cathode are .inserted'in the bath. Current is passed through the R wherein R is lower alkyl, lower cycloalkyl, aralkyl in which the alkylene moiety is methylene or vinylene and in which the aryl moiety is of the benzene or naphthalene series, hydrogen, or a radical of the formula:

wherein R and R, are independently selected from hydrogen, lower alkyl, lower cycloalkyl, aryl of the benzene and naphthalene series and aralkyl in which the alkylene moiety is methylene and in which the aryl moiety is of the benzene or naphthalene series, the weight proportion of the piperidine in said mixture being about 20 to about 80 percent.

2. A zinc electroplating bath as claimed in claim 1 wherein the concentration of the inhibitor mixture is about 0.004 to 10 grams per liter of the bath.

3. A zinc electroplating bath as claimed in claim 2 wherein the concentration of the inhibitor mixture is about 0.03 to 0.2 gram per liter of the bath.

4. A zinc electroplating bath as claimed in claim 1 wherein the weight proportion of the piperidine in the inhibitor mixture is about 50 to 75 percent.

5. A zinc electroplating bath as claimed in claim 1 wherein R is and R, is lower alkyl.

6. A zinc electroplating bath as claimed in claim 1 wherein R and R are both aryl radicals of the benzene series.

7. A zinc electroplating bath as claimed in claim 5 wherein the piperidine is lacetylethyl-4-hydroxy-4-methyl-S-acetylpiperidine and the thiourea is N,N'-di(p-tolyl)thiourea.

8. A zinc electroplating bath as claimed in claim 1 which contains in solution about 250 to 350 grams per liter of bath of zinc fluoborate, about 20 to 40 grams per liter of bath of ammonium chloride, about to 30 grams per liter of bath of ammonium fluoborate, an amount of ammonium hydroxide sufficient to provide a bath pH of about 3.0 to 3.5 and from about 0.03 to 0.2 gram per liter of bath of the mixture of the piperidine and the thiourea wherein the weight proportion of the piperidine is about 50 to 75 percent.

9. In the process of electroplating zinc on a ferrous metal cathode from an aqueous acidic zinc fluoborate bath employing a zinc anode, the improvement which consists in electroplating from a bath containing as an ironand metal-oxide deposition inhibitor a small effective amount of a mixture of a. a piperidine of the formula:

l /CH3 N wherein R is lower alkyl, lower cycloalkyl, aralkyl in which the alkylene moiety is methylene or vinylene and in which the aryl moiety is of the benzene or naphthalene series, hydrogen or a radical of the formula:

and R, is lower alkyl, aryl of the benzene or naphthalene series or aralkyl in which the alkylene moiety is methylene or vinylene and in which the aryl moiety is of the benzene or naphthalene series, and

b. a thiourea of the formula:

wherein R and R are independently selected from hydrogen, lower alkyl, lower cycloalkyl, aryl of the benzene or naphthalene series or aralkyl in which the alkylene radical is methylene and in which the aryl moiety is of the benzene or naphthalene series, the weight proportion of the piperidine in said mixture being about 20 to about percent.

10. An electroplating process as claimed in claim 9 wherein the concentration of the inhibitor mixture is about 0.004 to 10 grams per liter of the bath.

11. An electroplating process as claimed in claim 10 wherein the concentration of the inhibitor mixture is about 0.03 to 0.2 gram per liter of the bath.

12. An electroplating process as claimed in claim 9 wherein the weight proportion of the piperidine in the inhibitor mixture is about 50 to 75 weight percent.

13. An electroplating process as claimed in claim 9 wherein R is and R, is lower alkyl.

14. An electroplating process as claimed in claim 9 wherein R and R are both aryl radicals of the benzene series.

15. An electroplating process as claimed in claim 13 wherein the piperidine is lacetylethyl-4-hydroxy-4-methyl-5- acetyl-piperidine and the thiourea is N,N-di(p-tolyl)thiourea.

16. An electroplating process as claimed in claim 9 wherein the current density is between about 700 and about 3,000 amperes per square foot of anode.

17. An electroplating process as claimed in claim 16 wherein the current density is between about 1,000 and about 2,000 amperes per square foot of anode.

18. An electroplating process as claimed in claim 16 wherein about 1 to 5 percent of the current normally delivered to the zinc anode is shunted continuously or semi-continuously through an inert carbon anode inserted in the bath while maintaining the bath efficiency at about percent.

19. An electroplating process as claimed in claim 18 wherein about 2 to 5 percent of the current is shunted through an inert carbon anode.

20. An electroplating process as claimed in claim 19 wherein the bath contains in solution about 250 to 350 grams per liter of bath of zinc fluoborate, about 20 to 40 grams per liter of bath of ammonium chloride, about 10 to 30 grams per liter of bath of ammonium fiuoborate, an amount of ammonium hydroxide to provide an initial bath pH of about 3.0 to 3.5 and from about 0.03 to 0.2 grams per liter of bath of the mixture of the piperidine and the thiourea wherein the weight proportion of the piperidine is about 50 to about 75 percent, the electroplating being carried out at a temperature of about to F.

21. Ferrous metal plated with zinc in accordance with the process of claim 9. 

2. A zinc electroplating bath as claimed in claim 1 wherein the concentration of the inhibitor mixture is about 0.004 to 10 grams per liter of the bath.
 3. A zinc electroplating bath as claimed in claim 2 wherein the concentration of the inhibitor mixture is about 0.03 to 0.2 gram per liter of the bath.
 4. A zinc electroplating bath as claimed in claim 1 wherein the weight proportion of the piperidine in the inhibitor mixture is about 50 to 75 percent.
 5. A zinc electroplating bath as claimed in claim 1 wherein R is and R1 is lower alkyl.
 6. A zinc electroplating bath as claimed in claim 1 wherein R2 and R3 are both aryl radicals of the benzene series.
 7. A zinc electroplating bath as claimed in claim 5 wherein the piperidine is 1-acetylethyl-4-hydroxy-4-methyl-5-acetyl-piperidine and the thiourea is N,N''-di(p-tolyl)thiourea.
 8. A zinc electroplating bath as claimed in claim 1 which contains in solution about 250 to 350 grams per liter of bath of zinc fluoborate, about 20 to 40 grams per liter of bath of ammonium chloride, about 10 to 30 grams per liter of bath of ammonium fluoborate, an amount of ammonium hydroxide sufficient to provide a bath pH of about 3.0 to 3.5 and from about 0.03 to 0.2 gram per liter of bath of the mixture of the piperidine and the thiourea wherein the weight proportion of the piperidine is about 50 to 75 percent.
 9. In the process of electroplating zinc on a ferrous metal cathode from an aqueous acidic zinc fluoborate bath employing a zinc anode, the improvement which consists in electroplating from a bath containing as an iron- and metal-oxide deposition inhibitor a small effective amount of a mixture of a. a piperidine of the formula:
 10. An electroplating process as claimed in claim 9 wherein the concentration of the inhibitor mixture is about 0.004 to 10 grams per liter of the bath.
 11. An electroplating process as claimed in claim 10 wherein the concentration of the inhibitor mixture is about 0.03 to 0.2 gram per liter of the bath.
 12. An electroplating process as claimed in claim 9 wherein the weight proportion of the piperidine in the inhibitor mixture is about 50 to 75 weight percent.
 13. An electroplating process as claimed in claim 9 wherein R is and R1 is lower alkyl.
 14. An electroplating process as claimed in claim 9 wherein R2 and R3 are both aryl radicals of the benzene series.
 15. An electroplating process as claimed in claim 13 wherein the piperidine is 1-acetylethyl-4-hydroxy-4-methyl-5-acetyl-piperidine and the thiourea iS N,N''-di(p-tolyl)thiourea.
 16. An electroplating process as claimed in claim 9 wherein the current density is between about 700 and about 3,000 amperes per square foot of anode.
 17. An electroplating process as claimed in claim 16 wherein the current density is between about 1,000 and about 2,000 amperes per square foot of anode.
 18. An electroplating process as claimed in claim 16 wherein about 1 to 5 percent of the current normally delivered to the zinc anode is shunted continuously or semi-continuously through an inert carbon anode inserted in the bath while maintaining the bath efficiency at about 100 percent.
 19. An electroplating process as claimed in claim 18 wherein about 2 to 5 percent of the current is shunted through an inert carbon anode.
 20. An electroplating process as claimed in claim 19 wherein the bath contains in solution about 250 to 350 grams per liter of bath of zinc fluoborate, about 20 to 40 grams per liter of bath of ammonium chloride, about 10 to 30 grams per liter of bath of ammonium fluoborate, an amount of ammonium hydroxide to provide an initial bath pH of about 3.0 to 3.5 and from about 0.03 to 0.2 grams per liter of bath of the mixture of the piperidine and the thiourea wherein the weight proportion of the piperidine is about 50 to about 75 percent, the electroplating being carried out at a temperature of about 170* to 190* F.
 21. Ferrous metal plated with zinc in accordance with the process of claim
 9. 